Memorize these keyboard shortcuts, each of which activates a different editing mode: S for Sectors, L for Linedefs, V for Vertices, T for Things and W for 3D edit. Press S now. Right click somewhere, then drag the mouse right. You'll see a number along the line it creates: that's the length of the sector side. Left click at 256, then drag down for 256. Repeat until you have a square, finishing with a left-click upon the point you started at.